1. CSI-SF: Estimating Wireless Channel State Using CSI Sampling & Fusion
Riccardo Crepaldi∗, Jeongkeun Lee†, Raul Etkin†, Sung-Ju Lee†, Robin Kravets
∗University of Illinois at Urbana-Champaign
†Hewlett-Packard Laboratories
Improving Performance in High Speed Wireless
Channel Quality Metrics
How do systems describe the channel quality?
Pros
Finer granularity compared to SNR
Fading and phase shift information for each subcarrier
Tighter bound with channel performance [Halperin’10]
Cons
CSI is significantly larger than SNR
CSI Describes a specific MIMO configuration
CSI for multiple MIMO configurations must be measured for a complete channel characterization
PHY Layer enhancements:
MIMO technology
OFDM
Channel Bonding
Adaptive protocols for:
Rate adaptation
Channel selection
Optimal antenna scheme selection
Best AP selection
Choices based on:
Channel quality RF
AGC
FFT
Chan Estimation
MIMO Decoding
Demap
[N Input streams]
[M spatial streams] RF
AGC
FFT
Chan Estimation
Demap RF
AGC
FFT
Chan Estimation
Signal-to-Noise Ratio (SNR):
One scalar value per packet
Accurate only for narrow-band channel
Channel State Information (CSI):
M x N x W complex matrix
W= Number of OFDM subcarriers
Current n devices:
M=3, N=3
W=56 (20MHz) or W=114 (40MHz)
TX: M parallel data streams
RX: N receivers
What MIMO configuration should I choose?
The Channel quality is…
Why CSI?
CSI Matrix Combining
CSI-SF enables estimation of uninvestigated channel configurations
Z(t)
Bitstream
Encoding
Symbol Mapping
X(t)
Spatial Mapping (Q) RF
Channel
(H) RX
Y(t)
Application of CSI-SF
A real networking problem: Rate Adaptation
Metrics: SNR, effective SNR, estimated effective SNR
What’s a good metric?
Small or no gaps
Monotonic behavior
CSI
Ex: Estimate of a 2x3 configuration using two 1x3 received packets
2x3 – 20MHz
Large gaps
Non-monotonic beavior
Packet 1 (Tx: A, Rx: ABC)
Packet 2 (Tx: B, Rx: ABC)
Q1† 3 2 56 Q2 1
CSI1
CSI2
Estimated CSI
Power scale factor
2x3 – 40MHz
3x3 – 20MHz
eSNR
(effective SNR based on real samples)
e2SNR
(effective SNR based on CSI-SF estimates)
Quality of CSI-SF Estimate
SNR is not a good metric, eSNR and e2SNR are better
Compared to standard eSNR, e2SNR uses estimates instead of real samples thus limiting the overhead
Example of CSI-SF combining two 1x3 packets in a 2x3 packets
SNR [db]
OFDM subcarrier index
Packet 1: 1x3 – Ant A
Packet 1: 1x3 – Ant B
CSI-SF Reduces Overhead
Hypoyetical channel variation
Overhead required to rate control to converge using different metrics
Packet 3: 2x3 – Ant AB
Real CSI
Estimated CSI
Hewlett Packard Laboratories University of Urbana-Champaign, Department of Computer Science – Mobicom 2011